In case of a semiconductor memory device, particularly a mobile DRAM, a temperature compensated self refresh (TCSR) circuit is used to reduce a current consumption amount. The TCSR circuit is a circuit to change a self refresh period according to temperature, thereby reducing consumption current. In other words, by using the data retention time of a DRAM, the TCSR circuit shortens the self refresh period to increase a current consumption amount when the temperature is high, and lengthens the self refresh period to reduce a current consumption amount when the temperature is low.
In a digital TCSR circuit among the TCSR circuits, a digital thermometer is mounted within a semiconductor memory device, and a thermal code generated from the digital thermometer is coded to determine a self refresh period. The performance of a digital thermometer depends on how much corresponding thermal codes to the internal temperature of a DRAM are generated.
In case of a mobile DRAM, an input pad and a TQ pad are located on a side, and a DQ pad is located on the other side. Accordingly, a method of outputting a thermal code using a DQ pad in a semiconductor memory device has a problem that loading becomes large since the thermal code should be transmitted from an input pad to a DQ pad.
In order to improve this, there is used a method of outputting a thermal code using a TQ pad which is located on the same side with an input pad. In case of outputting a thermal code in this manner, there is an advantage that the number of transmission lines required for a thermal code transmission can be decreased, thereby reducing an area.
On the other hand, there is a test mode for measuring a thermal code being outputted through a TQ pad. This test mode is progressed in such a manner that three test mode signals are applied to output one thermal code, and therefore 18 test mode signals should be inputted to output a 6-bit thermal code generally being used.
Since test mode signals should be manually inputted by a tester, there occurs, at most, a several seconds of delay when outputting a thermal code. Accordingly, it is difficult to confirm the outputted thermal code at once using an oscilloscope.
In addition, it is difficult to judge whether or not the outputted thermal code is an accurate code unless test mode signals corresponding to a thermal code is correctly strobed. In particular, it becomes more difficult to judge whether or not the outputted thermal code is correct when a phase between thermal codes being sequentially outputted is identical.